Hydroxyacetic acid can be synthesized by the carbonylation of formaldehyde and water in an organic acid, usually hydroxyacetic, with sulfuric acid as a catalyst at pressures between 6,000 and 10,000 psig, and temperatures of 210.degree. to 240.degree. C., preferably 220.degree. C. The carbonylation process is well known and is described in U.S. Pat. Nos. 2,153,064; 2,152,852 and 2,037,654. Conversion of the formaldehyde to hydroxyacetic acid is approximately 75%. The largest loss of formaldehyde is due to by-product formation; for example, formic acid.
This process produces a crude hydroxyacetic acid which must be purified prior to use or sale. Presently the crude acid can be purified in a four-step process as disclosed in U.S. Pat. No. 3,859,347. It is first treated with granulated, activated carbon for decolorization, then treated in a weak anion resin column to remove the sulfuric acid, subjected to live steam stripping to remove low-boiling impurities and finally treated with a cation exchange resin to remove metals present, e.g., iron or copper.
More specifically, crude hydroxyacetic acid is produced as expressed by the formula: ##STR1## A typical liquid mixed feed containing 10.9% formaldehyde, 23.9% water, 1.3% sulfuric acid and 63.9% organic acids, preferably hydroxyacetic acid, and using typical reaction conditions results in about a 75% conversion of formaldehyde to crude hydroxyacetic acid. The liquid mixed feed is a 1 to 3 ratio of 58% formaldehyde to recycled crude acid product. An excess of carbon monoxide is used to assist in the complete conversion of the formaldehyde. The water contributed to the mixed feed by the addition of the aqueous formaldehyde accounts for the presence of only approximately 8.0% of the water content. The difference between the 23.9% water content of the feed and the 8.0% is due to the water content of the recycle stream for the following reasons:
(a) In 58% aqueous formaldehyde the water to formaldehyde molar ratio is 1.2 to 1; therefore, there is an excess of 0.2 mole of water for each mole of formaldehyde based on the stoichiometry of the reaction. PA1 (b) The product of synthesis is not free hydroxyacetic acid but a mixture of free hydroxyacetic acid and hydroxyacetic acid self-ester in a molar ratio of at least 0.7 to 0.3. This represents at least 0.3 mole of water unused in the reaction. The molar ratio of water to hydroxyacetic acid in the recycle stream because of (a) and (b) is about 0.5, making the water content 10.6% by weight. When this recycle stream is added to 58% aqueous formaldehyde in a weight ratio of 3 to 1, the resulting composite feed contains at least 18.5% water. PA1 (c) Methyl formate and methoxyacetic acid, two by-products of hydroxyacetic acid synthesis, are formed by reactions having a water to formaldehyde molar ratio requirement of only 0.5 to 1, thereby increasing the water content of the recycle stream: ##STR2## The sum of the effects described in (a), (b) and (C) accounts for the approximately 23.9% water in the feed a water to formaldehyde molar ratio in the mixed feed of 3.0 to 1 to 3.6 to 1. PA1 (a) a composite feed with a water to formaldehyde molar ratio of about 1.3 to 1, PA1 (b) a product after dilution to about 70% hydroxyacetic acid meeting the specification for formic acid concentration, and PA1 (c) a light yellow product containing substantially no unhydrolyzable hydroxyacetic acid.
The hydroxyacetic acid made by the above carbonylation and purification processes is typically a light yellow, 70% aqueous solution and of the following composition in weight percent.
______________________________________ Total Acid as HAA (%) 70.0-72.0 Free Acid as HAA (%) min. 62.4 Formic Acid (%) max. 0.45 Ash (%) max. 0.30 Suspended Matter by Volume (%) max. 0.015 Color (Gardner) max. 5 Iron as Fe (mg/kg) max. 10 Copper as Cu (mg/kg) max. 5 Chloride as Cl (mg/kg) max. 10 ______________________________________
Hydroxyacetic acid is a useful commercial acid which is typically used to remove milkstone, to polish metals and remove corrosion from pipes.